Absorption type infrared gas analyzers, which qualitatively or quantitatively analyze gas species contained in a sample gas based on infrared absorption by the gas species, are widely used in various technical fields because of their excellent selectivity and high sensitivity. The configuration and the operation principle of a conventional absorption type double beam infrared gas analyzer is explained, for example, with reference to FIG. 7. In FIG. 7, infrared radiation from an infrared radiation source 3 is chopped by a rotating chopper 2 to an infrared beam train flashing with a predetermined frequency. The beam train is divided into two beams by a distributor cell 4. One of the divided beams is led as a measuring beam 11 to a measuring cell 13 and the other divided beam is led as a reference beam 12 to a reference cell 14. The measuring cell 13 is comprised of infrared windows 5, 6 and inlet pipes 17, 18 through which a sample gas containing a component gas to be analyzed is introduced into or ejected from the measuring cell 13. The measuring beam 11 is absorbed in the measuring cell 13 depending on the content of the component gas to be analyzed.
The reference cell 14 is comprised of infrared windows 7, 8. A gas which does not absorb the infrared beam, nitrogen gas for example, is enclosed in the reference cell 14. The beams 11 and 12 which have passed through the cells 13 and 14 respectively are introduced into a gas enclosing type detector 20. The detector 20 includes the first and the second expansion cells 15 and 16 on which infrared windows 9 and 10 are respectively installed. A same species of gas with the gas to be analyzed is enclosed in the detector 20. The beam 11 impinges into the first expansion cell 15 through the infrared window 9 and the reference beam 12 impinges into the second expansion cell 16 through the infrared window 10. The detector 20 further includes a gas flow channel 19 which connects the expansion cells 15 and 16 to each other. A gas flow is originated across the channel 19 by pressure variation difference originated as a result of infrared absorption difference between the expansion cells 15, 16 corresponding to the content of the gas to be analyzed contained in the sample gas.
Thermo-elements 23, 24 in a thermal sensor 21 are shown in FIGS. 8(A), (B) and (C). FIG. 8(A) shows a front view of the thermo-elements 23 and 24, which are formed into two flat grids made of electrically conductive material such as metal or ceramics. The illustrated example of the thermo-element 23 or 24 is comprised of a resistor (hatched portion) fabricated by forming narrow interdigital slits by etching on a rectangular nickel foil. The thermo-elements 23 and 24 are arranged one over another with a narrow spacing in between as shown in FIG. 8(B).
FIG. 8(B) shows a sectional view taken along A--A of FIG. 8(A) of the thermo-elements 23 and 24. The hatched portions of FIG. 8(B) show portions of the above described zigzag train of the electrically conductive foil, the resistance of which varies sharply with temperature. The thermo-element 23 is placed on the left hand side and the thermo-element 24 is positioned in close proximity to the element 23. The corresponding central portions of the thermo-elements 23, 24 are formed as an opening which is arranged perpendicularly in the channel 19 which connects the first and second expansion 15 and 16 to each other. The thermo-elements 23 and 24 are respectively connected to resistors 27 and 28 to form a Wheatstone bridge circuit. The thermo-elements 23, 24 are heated up above the room temperature by electric power supplied to the Wheatstone bridge circuit. The thermo-elements 23, 24 are positioned in such close proximity as thermal coupling occurs between the elements 23, 24.
When a gas flow occurs through the channel 19 in response to pressure difference between the first and second expansion cells 15 and 16, the gas flow varies temperature distribution around the thermo-elements 23, 24. The Wheatstone bridge circuit detects the temperature distribution variation as an indication of the pressure difference.
The prior art infrared gas analyzer comprised of the Wheatstone bridge circuit consisting of two thermo-elements and two resistors is not sensitive enough to analyze low concentration gas species since the Wheatstone bridge circuit utilizes only the resistance change of the two thermo-elements. The infrared gas analyzer according to the prior art has solved this problem with an elongated measuring cell or with an infrared radiation source of high radiance. However, the elongated cell hinders down sizing of the optical system, and the high radiance radiation source requires additional thermal means, including a radiator, since the high radiance radiation source is realized by a high source temperature, which increases the size and cost of the infrared gas analyzer.
In view of the foregoing, an object of the present invention is to provide an infrared gas analyzer with improved sensitivity, small size and low cost which facilitates stable measurement of a gas of low content.